The present invention relates to a rubber reinforcing fiber cord, a method of manufacturing the same, and a radial pneumatic tire for a passenger car using the same. More specifically, the present invention relates to a rubber reinforcing fiber cord in which silk fibroin fibers are made usable for reinforcement of an automobile pneumatic tire and the like, a method of manufacturing the same, and a radial tire for a passenger car using the same.
For reinforcing fiber cords of rubber products including a pneumatic tire, synthetic fibers of nylon, polyester or the like based on petroleum resources are generally used. However, upon disposal of synthetic fibers, even if buried in the ground, they are never decomposed, and remain as environmental pollutants. Additionally, if incinerated, the synthetic fibers generate hazardous gas, and therefore become sources of environmental pollution as well. Therefore, as an environmental protection measure, it has been desired that materials made from non petroleum resources be used for the rubber reinforcing fiber cords.
Conventionally, in rubber reinforcing fiber cords, rayon fibers made from wood have been long known as materials made of non petroleum resources. Even today, rayon fibers are used for carcass layers of a part of pneumatic tires of passenger cars. However, since rayon fibers require use of toxic substances such as carbon disulfide in a manufacturing process therefor, it is necessary that this carbon disulfide should be strictly controlled in the manufacturing process in order to prevent the carbon disulfide from bringing about environmental pollution. Additionally, because forests should be logged for raw material procurement of rayon fibers, there is a problem that the raw material procurement thereof leads to destruction of the global environment. Accordingly, with respect to materials used for carcass layers especially for radial pneumatic tires of passenger cars, replacement of rayon fibers by polyester fibers has been in progress.
In contrast to rayon fibers as described above, silk is a biological resource, and if buried in the ground, is eaten by bacteria and disappears. Therefore, silk attracts attention as a material which does not involve a problem of environmental destruction. However, examples in the past which utilized silk strings for rubber reinforcing fiber cords can be found only with respect to bicycle tires, for instance, in descriptions in “Pneumatic Tire” (Henry C. Pearson, 1992, page 172) and in the paragraph 0025 of Japanese patent application Kokai publication No. Hei 11-301208. Nevertheless, there cannot be found an example utilizing silk strings in an automobile tire which bears a considerably large load as compared to a bicycle tire.
As one of the reasons why there is no example utilizing silk for a reinforcing fiber cord of an automobile tire used under such a severe condition, it can be considered that adhesion of silk strings to rubber has been insufficient. For example, there has been disclosed utilization of a natural-rubber based adhesive agent, a chloroprene based adhesive agent or the like as a method of adhering silk fibers to rubber in Patent Document 1. However, these adhesive agents not only cannot secure adhesion to rubber sufficient for a reinforcing fiber cord of an automobile tire used under the severe condition, but also are not favorable to the environment because organic solvents are used therein. Additionally, since silk fibroins are made of protein because of a characteristic intrinsic to silk, there can be cited another problem that silk fibroins have inferior thermal stability and low thermal resistance. Moreover, a surface of silk is covered with sericin which is a water-soluble protein, and it is difficult to completely remove this sericin through a refinement utilized industrially. The remaining sericin is assumed to be a factor of adhesion deterioration. Furthermore, there can be cited still another problem that the remaining water-soluble sericin facilitates acceleration of moisture absorption, and is likely to incur strength deterioration due to moisture absorption. These problems are assumed to be the reasons why silk has not been utilized for reinforcing fiber cords in rubber products, such as an automobile tire, which are acted upon by large loads and involve heat generation.